A magnetic recording medium composed of a non-magnetic support having thereon a magnetic layer containing ferromagnetic particles dispersed in a binder is generally used as a magnetic recording medium for audio recording, video recording or for computers. Audio tapes and video tapes are in many cases installed in a cassette for use.
Audio tapes installed in a cassette are generally used and stored under conditions having comparatively moderate temperature changes. However, with the development of car stereos, audio tapes have recently been used and stored in a car. When a car is parked directly in the sunshine, the temperature inside of the car becomes extraordinary high. In accordance with a testing method of car parts at a high and a low temperature (JIS-D-0204), it is reported that the maximum temperature sometimes reaches 104.degree. C. within a car at a daytime in summer. When an audio tape with a cassette equipped with a radio is used outdoors, the tape is in many cases allowed to stand at a comparatively high temperature. Also, frequently video tape installed in a portable video tape recorder or a video camera are used outdoors and as a result those tapes are allowed to stand also at a comparatively high temperature. When the tapes are used outdoors, they are liable to be exposed to relatively dusty conditions.
When a conventional magnetic recording medium is allowed to stand at such a high temperature, the tape often shrinks and curls, with one tape edge shrinking more and as a result, the tape deforms just like an arch in the lengthwise direction, or a hub is extruded by the winding force caused by the shrinkage of a tape. When a tape deforms as described above, upon reproducing, output levels are greatly varied and reproduction is unfaithful and running cannot be continued. Particularly when a magnetic recording medium having a thin support (for example, a long-play audio compact cassette tape such as "C-80", "C-90" or "C-120", and a long-play video cassette tape is used, troubles such as an extrusion of the hub tend to occur, because the tape to be wound is very long, and is wound around a hub so many times. Also when tapes are used outdoors under dusty conditions as described above, troubles such as drop out caused by dust and contaminants are liable to occur. Therefore, more excellent heat-resistance and running durability has been required for audio tapes and video tapes.
Carbon black is conventionally contained in the magnetic layer of such a magnetic recording medium for preventing the static electrification of the magnetic layer. An average primary particle diameter of carbon black is generally from 10 to 150 m.mu. (milli micron). It is generally considered that when carbon black having a small particle size is used, the surface of the thus obtained magnetic layer becomes smooth, but running durability and heat resistance become poor. On the other hand, when carbon black having a large particle size is used, running durability becomes excellent but surface smoothness becomes poor, which results in undesirably deteriorating electromagnetic properties and particularly S/N in the case of a video tape. For solving the above problem, it has been proposed in Japanese Patent Publication Nos. 9041/79 and 20203/78 and Japanese Patent Application (OPI) No. 218039/83 (the term "OPI" as used herein means a "published unexamined Japanese patent application"), that carbon black having different average primary particle sizes be added in a magnetic layer when the magnetic layer is a single layer, but a magnetic recording medium having both satisfactory running durability and electromagnetic properties cannot be obtained.
On the other hand, when a magnetic layer is a multiple layer having at least two layers, a method for preparing a magnetic recording medium having excellent running durability as well as excellent electromagnetic properties is disclosed in Japanese Patent Application (OPI) No. 200425/83. That is, the magnetic layer is composed of two magnetic layers, and only the second magnetic layer (upper layer) contains carbon black to improve running durability and heat resistance. The first magnetic layer (under layer) does not contain carbon black so as to increase packing density of magnetic particles in magnetic layer, whereby electromagnetic properties are improved. However, the above described magnetic tape has such small carbon black particles (i.e., average primary particle diameter is 30 m.mu.) contained only in the upper layer and satisfactory running durability and heat resistance cannot yet be obtained.
Accordingly, in this material when electromagnetic characteristics are excellent, running durability is insufficient, and heat resistance is not adequate, and satisfactory performances cannot be obtained.